A TeX engine in the initial state ('IniTeX') knows only the primitives and a very small number of pre-defined settings. For almost all practical use you want a set of macros on top of this. That can be achieved by doing
and then typesetting them in interactively, but is much more practically achieved by having them saved into a file. You could read that file at the start of every run you did, so start your input with
In most real cases this standard file will end with the primitive
\dump: that ends the TeX run and saves the current state of TeX's memory so it can be reread later. Your 'real' document then does not ever need to worry about those pre-defined macros: they are loaded at start up from the dump file. Historically, you would have told TeX to read a dump using
so for example
for the LaTeX format (
latex.fmt). However, you don't need to do that as the command
latex is available as more-or-less equivalent to the same thing. (There are technical differences, and importantly a modern
latex will be more akin to
pdftex -output-format=dvi &latex.)
Note that a format file is a memory dump. As such, it is only usable by exactly the same binary that made it: same OS, same version, etc. As such, format files are not distributed, but are created on the system where they are used.
If you look at the LaTeX Team SVN you will find a variety of files concerned with the process of building and testing LaTeX for release. Our build system,
l3build, does make format files for local use in testing. However, this is not the same as making a format file for a user. Indeed, the official release of LaTeX, like most other TeX software, is via CTAN, and we do not send the build system material there (users do not need it).
Most end users don't get material directly from CTAN in any case: they get LaTeX as part of a TeX system, likely TeX Live or MiKTeX. These systems do the format building and saving automatically, and users therefore very rarely need to be concerned with the detail.